An incredible shrinking material

10:53 PM, 7th November 2011

Heat causes the atoms in ScF3 to vibrate, as captured in this snapshot from a simulation. Fluorine atoms are in green while scandium atoms are in yellow.

PASADENA, US: They shrink when you heat'em. Most materials expand when heated, but a few contract. Now engineers at the California Institute of Technology (Caltech) have figured out how one of these curious materials, scandium trifluoride (ScF3), does the trick - a finding, they said, that will lead to a deeper understanding of all kinds of materials.

The researchers, led by graduate student Chen Li, published their results in the November 4 issue of Physical Review Letters (PRL).

Materials that don’t expand under heat aren’t just an oddity. They’re useful in a variety of applications - in mechanical machines such as clocks, for example, that have to be extremely precise. Materials that contract could counteract the expansion of more conventional ones, helping devices remain stable even when the heat is on.

“When you heat a solid, most of the heat goes into the vibrations of the atoms,” explained Brent Fultz, Professor of materials science and applied physics and Co author of the paper. But because crystal structures are complicated, scientists have not been able to clearly see how heat, could lead to contraction.

But in 2010 researchers discovered negative thermal expansion in ScF3, a powdery substance with a relatively simple crystal structure. To figure out how its atoms vibrated under heat, Li, Fultz, and their colleagues used a computer to simulate each atom's quantum behavior. The team also probed the material's properties by blasting it with neutrons at the Spallation Neutron Source at Oak Ridge National Laboratory (ORNL) in Tennessee.

The results paint a clear picture of how the material shrinks, the researchers said. You can imagine the bound scandium and fluorine atoms as balls attached to one another with springs. The lighter fluorine atom is linked to two heavier scandium atoms on opposite sides. With temperature, all atoms jiggle and because of the linear arrangement, fluorine vibrates more in directions perpendicular to the springs. With every shake, the fluorine pulls the scandium atoms toward each other. Since this happens throughout the material, the entire structure shrinks.

The surprise, the researchers said, was that in the large fluorine vibrations, the energy in the springs is proportional to the atom’s displacement - how far the atom moves while shaking - raised to the fourth power, a behaviour known as a quartic oscillation. “A pure quartic oscillator is a lot of fun,” said Fultz. “Now that we’ve found a case that’s pure, I think we know where to look for it in many other materials.” Understanding quartic oscillator behaviour will help engineers design materials with unusual thermal properties.

The other authors of the PRL paper, “The structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3,” are former Caltech Postdoctoral Scholars Xiaoli Tang and J Brandon Keith; Caltech graduate students Jorge Munoz and Sally Tracy; and Doug Abernathy of ORNL. The research was supported by the Department of Energy.